An automatic analysis device, which irradiates a reaction solution obtained by reaction of a sample with a reagent with light, and calculates absorbance from the amount of transmitted light that has passed through the reaction solution in accordance with the Lambert-Beer's law, and then quantitatively determines the concentration of a substance in the sample from the amount of change of absorbance that occurred in a given period of time, is widely used (e.g., see Patent Literature 1). In such a device, a number of cells each holding a reaction solution are circumferentially arranged on a reaction disc. An absorbance measuring unit is arranged around the reaction disc that is rotationally driven, so that time-series data on the amount of light that has passed through the reaction solution (the amount of transmitted light) is acquired for about 10 minutes at intervals of once every 15 seconds. The time-series data herein is referred to as reaction process data. Absorbance is calculated from the amount of transmitted light, and the concentration of a substance is quantitatively determined from the amount of change of absorbance that occurred in a given period of time.
Reactions that are measured by an automatic analysis device include two types: a color reaction that uses a substrate and an enzyme, and immune agglutination that uses an antigen and an antibody. A method for quantitatively determining the concentration through a color reaction is referred to as a biochemical analysis. Examples of test items of the biochemical analysis include LDH (lactate dehydrogenase), ALP (alkaline phosphatase), and AST (aspartate-oxoglutarate aminotransferase). A method for quantitatively determining the concentration of a substance through immune agglutination is referred to as an immune assay. Examples of test items of the immune assay include CRP (C-reactive protein), IgG (immunoglobulin), and RF (rheumatoid factor). The concentration of a substance, which is measured through immune agglutination, in the blood is relatively low. Therefore, a reagent used is required to be highly sensitive. Thus, latex particles, which are obtained by sensitizing antibodies as a sensitizer on the surface of a reagent, are often used as a reagent. The latex particles with sensitized antibodies on the surface will aggregate via a substance to be measured, thereby forming an aggregate. The aggregate will grow with time. The higher the concentration of the substance, the larger the aggregate that is obtained after a given period of time has elapsed. According to the measurement methods so far, a reaction solution in which an agglutination reaction has progressed is irradiated with light, and the amount of transmitted light that has passed through the reaction solution is measured to quantitatively determine the concentration of the substance to be measured.
However, in recent years, a further increase in sensitivity is desired for the latex immunoassay, and attempts have been made to measure not transmitted light but scattered light. For example, a system (Patent Literature 2) is disclosed that measures absorbance and scattered light at the same time by separating the transmitted light from the scattered light using a diaphragm, for example.